Automatic frequency control system



Aug. 14, 1951 J. HALPERN ET AL AUTOMATIC FREQUENCY CONTROL SYSTEM Filed June 25, 1945 FIG. I

SERVO AMPLIFIER PHAS E DETECTO R DIAPHRAGM assoum-oa l2 CRYSTAL I'll FREQUENCY FREQU E N C Y LDW-CORRECT-HIGH:

. INVENTORS. JULIUS HALPERN ROBERT V POUND ATTORNEY BY 9. EL

Patented Aug. 14, 1951 AUTOMATIC FREQUENCY CONTROL SYSTEM Julius Halpern, Brookline, and Robert V. Pound,

Cambridge, Mass, assignors, by mesne assignments, to the United States of America as represented by the Secretary of War Application June 23, 1945, Serial No. 601,118

2 Claims. 1

This invention relates in general to frequency control systems for thermionic tubes having cav-- ity resonators as resonant elements, and more particularly to systems wherein such control is automatic.

For some purposes in communication systems using ultra-high frequencies, the transmitting tubes used are magnetrons using cavity resonators as resonant elements. Magnetrons of this type have the properties of drifting in frequency as the tube is warming up, and having their frequency changed, or pulled, with a variation in loading. This frequency pulling is undesirable because it results in the receiver used in the communication system being improperly tuned to the transmitter frequency most of the time. A system commonly used to alleviate this difficulty is one in which the receiver is monitored as to tuning during the warming-up period of the magnetron, and the receiver bandwidth is mad wide enough to effectively cover any frequency pulling that may occur afterwards. This system is undesirable in that the former procedure is cumbersome and the latter procedure is expensive in relation to equipment and space requirements.

Among the objects of our invention, therefore,

are:

1. To provide a magnetron capable of being tuned over a range of frequencies.

2. To provide a control system for automatically keeping the frequency of the magnetron constant.

In accordance with the present invention there is provided a magnetron oscillator with a tuning means embodied therein. A portion of the magnetron output is fed into a reference cavity resonator, which is of such a size and shape as to be resonant at the desired magnetron system frequency. The magnetron operating frequency and the reference cavity frequency are compared and a D.-C. voltage is developed which is dependent upon the magnitude and sense of the difference between the two frequencies. This D.-C. voltage drives a servo system which moves a mechanical tuning means in such a direction as to reduce this frequency difference.

Our invention will best be understood with reference to the drawings, in which:

Fig. 1 is a functional diagram of the system as a whole according to our invention;

Fig. 2 is a graph of important waveforms occurring within the system; and

Fig. 3 is a circuit diagram of a phase detector which may be used in this system.

Referring now to the description of the appa- 2 ratus and more particularly to Fig. 1, ther is shown a magnetron l0 whose output is coupled into a transmission means II, shown for an example as a wave guide. A small portion of the output of the magnetron I0 is fed into a cavity resonator l2. An audio frequency source l3 feeds the windings of a coil l5, which is mechanically connected to a diaphragm l6 which makes up a portion of the walls of the cavity l2. The field from the current in coil 5 reacts on the field from permanent magnet l4 so as to cause the coil and therefore the diaphragm to vibrate. The output of cavity I2 is fed to a crystal ll, the rectified output of which, together with the A.C. output of source It! is fed into a phas detector l8 labeled as such. The output of phase detector i8 goesinto a servo-amplifier unit 19 which is labeled as such.

One embodiment of this system has a servoamplifier unit which has as its output a current which is fed to the motor 20 and controls its direction of motion. The motor drives a gear train 2| which moves a tuning plunger 22 for mechanically tuning the magnetron. It is to be understood, however, that any type of servo system may be used whereby the magnitude and sense of the movement of the tuning plunger 22 can be made to be dependent upon the magnitude and sign of the phase detector output voltage. In this figure, electrical circuits are shown in solid lines and mechanical circuits are shown in dashed lines. Arrows shown on the drawing represent the directions of power flow.

One circuit that may be used as phase detector I8 is shown in Fig. 3. It has two inputs-one, at point 30, is connected to the crystal I! as noted; the other, across points 3| and 32, goes to the A.C. source l3 as noted. The varying crystal voltage is fed through series condenser 33 to grids 34 and 35 of tubes 36 and 31 respectively. These grid voltages are developed across grid-leak resistor 38. One end 46 of resistor 38 is connected to the cathode of tube 31 through a parallel network consisting of condenser 39 and resistor 40. To the end 46 of resistor 38 is also connected the cathode of tube 36 through a parallel network of resistor 4| and condenser 42. The cathode of tube 36 is grounded. The A.C. voltage is fed to the primary 43 of transformer 44. The secondary 45 has one side going to the plate of tube 36 and the other sid going to the plate of tube 31. The secondary 45 is center-tapped and connected to one end 46 of resistor 38 through th power supply, marked B+. Thus the plate voltages of tubes 36 and 3! are out of phase. The out- 3 put is taken between the cathode of tube 3! and ground, as marked.

Referring now to the operation of the system, and more particularly to Fig. 1, the magnetron I8 is coupled into a transmission means ll, out of which is extracted a small amount of power to excite cavity l2. With diaphragm It in its quiescent position, the dimensions and shape of the cavity 42 are such that it resonates normally at the desired operating resonant frequency for the magnetron. Since the cavity is coupled to a crystal, the rectified output from the crystal as a function of cavity frequency, representing the voltage output from the cavity, is as shown in Fig. 2, curve 56, with the frequency labeled in being the quiescent cavity resonant frequency. The oscillations within the cavity, however, are being constantly frequency-modulated at a low frequency rate by the vibratin diaphragm. This modulation of the cavity frequency is represented by curve 5! when the magnetron frequency is lower than the cavity frequency, curve 52 when both frequencies are the same, and curve 53 when the magnetron is oscillating at a higher frequency than that to which the cavity is quiescently tuned. Curves 5f, 52 and 53 are shown in their correct relationships to the frequency axis of curve 56, and are labeled as to th magnitude of the magnetron frequency with respect to the quiescent cavity frequency.

Due to the shape of the curve 50 of crystal out put as a function of frequency of oscillations within the cavity, when the magnetron frequency is too low, as represented by curve 5!, the crystal current varies as shown in curve 5c. When the magnetron is oscillating at the correct frequency, the quiescent cavity frequency, as represented by curve 52, the crystal current output is as shown in curve 55. When the magnetron frequency is too high, as represented by curve 53, the crystal current output varies as shown in curve 55. Curves 5%, 55 and 56- are labeled as to whether the magnetron frequency is lower than, correct with respect to, or higher than, the quiescent resonant frequency of the cavity. By inspection of the curves of Fig. 2, it can be seen that when the magnetron frequency is low, the crystal current is in phase with the voltage from the A.-C. source; when the magnetron frequency is high, the crystal current is 180 out of phase with the voltage from the A.-C. source. When the magnetron is oscillating at the quiescent resonant frequency of the cavity, the crystal current has a frequency which is twice that of the A.-C. source.

How the crystal current and voltage from the A.-C. source affect the output of the phase detector [8 can be seen by referring first to Fig. 3 and then to Fig. 2. The varying crystal current is fed in the form of .a varying voltage to the two grids 34 and 35 of the tubes 36 and 37. The varying voltage on these two grids will be in phase at all times. The voltage from the A.-C. source is passed through transformer 44 and so appears on the two plates of the tubes 35 and 3'! 180 out of phase. Let us examine the case where the magnetron frequency is lower than the quiescent cavity frequency. Under this condition the the crystal current is in phase with the voltage from the A.-C. source. Let the polarization of the transformer windings be such that under this condition the grid voltage and plate voltage in tube 36 are in phase. This will cause a large current through tube 36 during the part of the cycle when the grid is going positively and a very small current through it when the grid is going negatively. The grid and the plate voltages of tubes 3'! are out of phase, so the current through tube 31 is smaller than that in tube 36 when the grid goes positively, but still is quite small when the grid is going negatively. The varying currents in tubes 36 and 31 are smoothed out by the comparatively large time constants of the resistance-capacitance networks in the cathode circuits of these tubes. In this case the average current in tube 35 is greater than the average current in tube 31 because of the large increase in tube current when both grid and plate went positively, and so the D.-C. output voltage'of the phase detector will be negative. Similarly, it can be seen that when the magnetron frequency is higher than the quiescent cavity resonant frequency, the crystal current output is 180 out of phase with the voltage from the A.-C. source, so the average current in tube 31 is greater than that in tube 36 and the D.-C. output voltage will be positive. When the magnetron frequency is correct, during one half-cycle of the A.-C. .voltage one tube will be carrying the most current, and during the other half-cycle the other tube will be carrying the most current, and as a result of the smoothing networks, the D.-C. output voltage is substantially zero.

Referring again to Fig. l, the output of the phase detector i 3 is fed into a servo-amplifier unit 53. The output current of this amplifier system is fed into the servo motor 2%. This motor drives a gear train 2% which varies the position of a tuning slug in the magnetron. The direction of the output current of the amplifier unit I!) and the direction of rotation of the servo motor 20, is determined by the polarity of the D.-C. input voltage to this unit. Thus, the system is arranged so that any difference between the magnetron operating frequency and the cavity quiescent frequency will produce a D.-C. voltage which will create a D.-C. current which drives the motor in such a direction as to vary the position of a slug within the magnetron so as to lessen the frequency difference.

It is understood that any type of servo system may be used as long as the direction of rotation of the motor is dependent upon the polarity of the D.-C. output voltage of the phase detector. Any type of mechanical system for tuning the magnetron may be applied practicably to this system, as long as the action of the tuning means is not too detailed mechanically.

While there has been described what is at present considered the preferred embodiment of the invention, it will be obvious to those skilled in the art that various changes and modifications could be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

We claim:

1. A radio frequency control system comprising a magnetron generator having tuning means, a cavity resonator including a diaphragm as a portion of a wall thereof, said resonator being quiescently tuned to the desired frequency of said generator, wave guide means to couple a portion of the output of said generator to said resonator, a reference frequency source, means coupling said diaphragm for vibration with said reference frequency source whereby the tuning of said resonator is periodically varied about said desired frequency in accordance with the reference frequency, a crystal rectifier disposed within said a pair of discharge devices, each at least having 1 cathode, grid and plate electrodes, a first resistance-capacitance parallel network in the cathode circuit of one of said devices, a second resistancecapacitance parallel network in the cathode circuit of the other of said devices, a common input circuit for the grids of said pair of devices, means to apply said control frequency to said common input circuit, means to apply said reference frequency in phase opposition to the respective plates of said pair of devices, and means to derive the differential voltage developed between said first and second networks to produce a control voltage, and means responsive to said control voltage and operable upon said generator tuning means to maintain the operating frequency of 1 said generator at the desired frequency.

2. A radio frequency control system comprising a magnetron generator having a mechanically operated tuning member, a cavity resonator including a diaphragm as a portion of a wall therea of, said resonator being quiescently tuned to the desired frequency of said generator, wave guide means to couple a portion of the output of said generator to said resonator, a reference frequency source, means coupling said diaphragm for vibration with said reference frequency source whereby the tuning of said resonator is periodically varied about said desired frequency in accordance with the reference frequency, a crystal rectifier disposed Within said resonator to derive a control frequency therefrom, a phase detector to compare the phase of said control frequency with said reference frequency and develop a control voltage whose polarity and magnitude is according to said comparison, the phase of said control frequency being dependent on the direction of deviation of the operating frequency of said generator from said desired frequency, said phase detector including a pair of discharge devices, each at least having cathode, grid and plate electrodes, a first resistance-capacitance parallel network in the cathode circuit of one of said devices, a second resistance-capacitance parallel network in the cathode circuit of the other of said devices, a common input circuit for the grids of said pair of devices, means to apply said control frequency to said common input circuit, means to apply said reference frequency in phase opposition to the respective plates of said pair of devices, and means to derive the difierential voltage developed between said first and second networks to produce a control voltage, a servo amplifier coupled to the output of said phase detector, a reversible motor responsive to the output of said servo amplifier, said reversible motor serving to drive said magnetron tuning member in accordance with said control voltage impressed upon said servo amplifier whereby the operating frequency of said generator is maintained at said desired frequency.

JULIUS HALPERN. ROBERT V. POUND.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,936,414 Stone Nov. 21, 1933 2,106,825 Bernarde Feb. 1, 1938 2,115,521 Fritz et a1. Apr. 26, 1938 2,144,222 Hollmann Jan. 17, 1939 2,167,201 Dallenbach July 25, 1939 2,233,263 Linder Feb. 25, 1941 2,241,937 Trevor May 13, 1941 2,250,532 Hansell July 29, 1941 2,294,942 Varian et al Sept. 8, 1942 2,348,986 Linder May 16, 1944 2,374,810 Jremlin May 1, 1945 2,393,284 Brown Jan. 22, 1946 2,404,568 Dow July 23, 1946 2,425,924 Crosby Aug. 19, 1947 FOREIGN PATENTS Number Country Date Great Britain Sept. 22, 1938 

